Photochromic optical articles, such as spectacle lenses, have been known for long. They include photochromic dyes which are able to darken and thus protect the eye from intense light when exposed to UV light. The wavelengths at which these photochromic dyes are activated usually range from 315 to 420 nm. Compared to conventional permanently tinted lenses, these photochromic lenses do not have the drawback of providing a low level of light transmission in a dark environment, which is prejudicial to a clear vision, and are thus better suited for driving, for instance. Among these photochromic lenses, some have a gradient profile. They are thus darker at the upper portion of the lens while having a lighter tone at the bottom, which is advantageous because they allow more light to be transmitted at the lower portion for activities involving close vision, such as reading. These gradient photochromic lenses are also considered fashionably appealing by many.
In order to obtain such gradient lenses, several methods have been proposed in the art. The method disclosed, for instance, in US 2008/0187760, consists in applying to the lens a layer of crosslinked organic polymeric matrix containing a photochromic dye in a gradient concentration.
Another method, disclosed for instance in GB-1,520,099 and U.S. Pat. No. 4,289,497, uses a uniform layer of photochromic dye coated by a non-uniform layer of UV absorbers which either has a variable thickness or contains a variable concentration of absorber. Since the UV absorber inhibits the photochromic effect, the latter is varied locally, which results in a tint gradient. The variation in the thickness of the absorbing layer may be obtained by gradual immersion of the lens in a bath containing the absorber, or by adjusting the withdrawal rate of the lens from the bath.
Another approach for producing an ophthalmic lens with spatially non-uniform photochromic properties has been disclosed in US 2012/0218512. This method comprises depositing an absorbent layer on a plastic film, which layer is able to transfer, when heated, the absorber printed therein to the film, then removing the absorbent layer by washing with a solvent to obtain a film containing an UV absorber distributed therein in a non-uniform manner, corresponding to the pattern previously printed. The film may then be affixed to a photochromic optical substrate.
These processes are not totally satisfactory.
Moreover, it would be useful to provide an optical system comprising both a gradient photochromic layer and an anti-reflection layer. This is because anti-reflection coatings have proven to be useful for providing the user with an improved contrast and an enlarged field of vision under the effects of glare. These anti-reflection coatings typically consist of a stack of dielectrics deposited under vacuum conditions. However, an anti-reflection coating is usually regarded as incompatible with a photochromic coating, since conventional anti-reflection coatings reject a significant part of UV light, hence inhibiting photochromic effects. Compromises have been developed so as to provide anti-reflection lenses with high transparency under low UV conditions and simultaneously photochromism under high UV conditions (see for instance U.S. Pat. No. 7,633,681 and U.S. Pat. No. 6,175,450). However, these compromises yield lenses tinted in a uniform manner, which thus do not provide any gradient of photochromic effect, and which are sometimes not totally clear in the absence of UV light.
U.S. Pat. No. 6,674,587 teaches a way to deposit a graded absorbing film upon a plastic substrate under vacuum by using a mask on the evaporation source. The absorbing material absorbs radiation in the UV and IR regions of the spectrum. The absorbing film is then coated with an anti-reflection coating to obtain an antiglare optical article. The method disclosed in this document is not applicable to optical articles such as ophthalmic lenses because it requires too many steps to be used industrially. Moreover, the optical article thus obtained would be exposed to a risk of delamination, which is not acceptable.
The inventors have now found that an optical article having a gradient photochromic effect and possibly a high transparency in the visible range, while exhibiting anti-reflective properties, could be obtained by conventional vacuum technology in a few steps, on an industrial scale, by means of a process which does not use UV absorbers but a specific interference coating design which reflects the wavelengths in the UV range so as to provide a gradient photochromic tint. This process may be conducted with conventional mass-production equipment.